1. Field of the Invention
This invention is related to a method for conductivity control of (Al,In,Ga,B)N.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary compounds, incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
Semiconductor optoelectronic devices typically rely on the transition of electrons between filled and unfilled electronic states, with the subsequent emission or absorption of photons of light. State-of-the-art optoelectronic devices, such as light emitting diodes (LEDs), laser diodes (LDs), or photovoltaic cells, incorporate a semiconductor diode junction. A diode junction results from a transition within or between regions of the semiconductor with differing n- and p-type carriers. An n-type semiconductor refers to a material with a net excess of free charge carriers within the conduction band of the semiconductor crystal. A p-type semiconductor refers to a material with a net excess of free charge carriers within the valence band of the semiconductor crystal. Therefore, the fabrication of optoelectronic semiconductor devices depends on the ability to intentionally modulate the concentration and type of free charge carrier within or between regions of the semiconductor crystal.
GaN and its alloys are most stable in the hexagonal wurtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the wurtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the wurtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on nonpolar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are crystallographically equivalent to one another so the crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN are the {11 20} family, known collectively as a-planes, and the {1 100} family, known collectively as m-planes. Unfortunately, despite advances made by researchers at the University of California, the assignee of the present invention, growth of nonpolar nitrides remains challenging and has not yet been widely adopted in the III-nitride industry.
Another approach to reducing or possibly eliminating the polarization effects in GaN optoelectronic devices is to grow the devices on semipolar planes of the crystal. The term semipolar planes can be used to refer to a wide variety of planes that possess two nonzero h, i, or k Miller indices, and a nonzero 1 Miller index. Some commonly observed examples of semipolar planes in c-plane GaN heteroepitaxy include the {11 22}, {10 11}, and {10 13} planes, which are found in the facets of pits. These planes also happen to be the same planes that the authors have grown in the form of planar films. Other examples of semipolar planes in the wurtzite crystal structure include, but are not limited to, {10 12}, {20 21}, and {10 14}. The nitride crystal's polarization vector lies neither within such planes or normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the {10 11} and {10 13} planes are at 62.98° and 32.06° to the c-plane, respectively.
In addition to spontaneous polarization, the second form of polarization present in nitrides is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al, In, Ga, B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to poor lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer latticed matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
Nakamura, et al. developed the first high quality p-type GaN material using magnesium (Mg) doping and a subsequent thermal annealing step. Both this technique and subsequent improvements by other researchers are described in references [1,2,3]. However, these techniques use GaN films grown along the polar [0001] direction. The advantage of using semipolar planes over c-plane nitrides is that the total polarization will be reduced. There may even be zero polarization for specific alloy compositions on specific planes. What is most important is that the polarization be reduced compared to that of c-plane nitride structures. Therefore, significant improvements may be made using semipolar growth orientations which alter the band structure, and therefore the conductivity, of the semiconductor crystal.
Conventional c-plane oriented nitride semiconductor crystals doped with Mg have p-type carrier concentrations approximately 100 times lower than the Mg dopant concentration due to the electronic band structure of the doped, polar nitride crystal. However, the present invention results in semipolar nitride semiconductor crystals with a p-type carrier concentration only a factor of 10 lower than the Mg dopant concentration. In other words, the doping concentration can be dramatically increased compared to that of polar nitride crystals. This enhanced Mg activation makes the present invention superior to conventional polar nitride p-type doping techniques. Another new feature of the invention results in p-type (Al,In,Ga,B)N films that exhibit both high hole concentrations >1018 cm−3, while at the same time maintaining Hall mobilities of 8-14 cm2/Vs. In other words, the carrier concentration and mobility can be simultaneously increased or maximized, and are higher than for polar nitrides. Traditional c-plane oriented p-type nitride semiconductor crystal exhibits much lower mobilities for equivalent hole concentrations. The resulting product of hole mobility and concentration results in a higher conductivity for (Al,In,Ga,B)N semiconductor crystal fabricated using this invention.
The present invention allows for p-type conductivity control in nitride (Al,In,Ga,B)N semiconductor crystals. Although p-type conductivity control has been previously reported for nitride semiconductors, the use of the semipolar and nonpolar planes to control the p-type conductivity in a nitride semiconductor has not been disclosed.